Machines for extruding or otherwise processing plastic materials, for example, have components such as rollers, worms or the like which must be continuously heated in order to prevent premature hardening of the polymeric mass. In other instances a rotating cylinder or drum has to be continuously cooled. Since the heating or cooling fluid is usually supplied by a stationary source, suitable seals must be provided at the inlet end and in most cases also at the outlet end of the channel system of the rotary member. Frequently, these inlet and outlet ends are constituted by lateral ports of a shaft integral with the member whose temperature is to be fluidically controlled, the ports communicating with respective compartments formed in an annular clearance between the shaft and a surrounding stationary coupling sleeve with the aid of sealing rings of rubber or the like carried by that sleeve. In order to minimize leakage, the rings must fit tightly on the shaft and may suffer damage when the sleeve is pushed onto the free shaft end during assembly. That risk is not completely avoided by tapering the shaft extremity, apparently because local peripheral stresses cannot be properly equalized.